The primary purpose of the proposed study is to determine if in patients with coronary heart disease (CHD) treatment with the antibiotic, azithromycin, decreases the rate of CHD events: CHD death, non-fatal myocardial infarction (heart attack), hospitalization for unstable angina (pain), and cardiac surgical procedures. Secondary objectives relate to better understanding the relationship between antibody titer, treatment, and outcome. Chlamydia pneumoniae is a bacteria that has been associated with human cardiovascular disease. The results of small trials which evaluated the use of azithromycin have indicated a possible effect of this agent in the secondary prevention of CHD. The evidence supporting this association provides a basis for initiation of further trials to assess treatment against C. pneumoniae on CHD outcomes.